A variety of commercial coating compositions are known for preparing metal surfaces to prevent corrosion and improve adhesion of paints or other coatings to the surface. For example, silane pretreatment coatings are used in commercial applications to provide anticorrosion properties to metal surfaces and/or to prepare the metal surfaces prior to painting operations. However, silane pretreatments are colorless when applied, are applied at very low coating weights, and are extremely difficult to detect by human visual inspection.
Methods for determining the presence of thin films on a substrate are known. Certain fluorescent dyes in metal film coatings have been used to determine whether or not the coating has been applied to the surface. Specifically, the fluorescent brightening agents stilbene and coumarin are added to a chrome-free metal coating and, after coating the metal with the coating, the metal is viewed under ultraviolet (UV) light and the presence of coating is detected by eye. The methods are useful in their ability to determine whether a coating has been applied to a substrate. The methods have been developed which involve quantitative determination of the thickness of the coating, an important quality control matter. Although possible, use of stilbene and coumarin in quantitative determinations of thickness of a coating is not preferred because it has been found that these compounds often do not display the required precision in their use in a calibrated system to determine coating thickness that is required in a commercially feasible coating thickness measurement system. The native fluorescence of the coating can interfere with measurement of fluorescent intensity of the coating.
Methods for determining thickness of transparent oil films on metal surfaces by detection of fluorescent compounds mixed in the oil are also known. However, due to the nature of oil films, these methods are not precise and, therefore, are not suitably reproducible for determination of the thickness of a dryable, dried, curable or cured film-forming coating on a substrate. Choice of dye is not important when the dye is used to measure oil film thickness. The layer of oil is typically not maintained on the surface of the substrate if additional layers of a coating are needed on the oil-covered substrate, such as a pre-coating a primer or a color coat.
Many of the organic dyes of the prior art are not readily soluble in waterborne pretreatments and thus, often affect the stability of the pretreatment. Addition of these organic dyes results in precipitation or gelation of the other components of the pretreatment, thereby making the pretreatment composition unstable. The poor shelf-life of the pretreatment affects the usability of these pretreatments because the precipitation or gelation results in non-uniform coating or the formation of voids or poorly coated surfaces. Poor corrosion resistance and poor adhesion of paints or other coatings to the metal substrate result when these unstable pretreatments are used. Accordingly, there is a need to be able to quickly detect the presence, uniformity and thickness of these pretreatments as an essential aspect of quality control.